SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
We would like to thank Dr. Hug for his comments regarding our recent article (2) that examined the estimation of EMGFT for the three superficial muscles of the quadriceps femoris during isometric leg extension. We agree that there are no definitive criteria with which to judge the precision of the estimation of EMGFT for various muscles and different modes of exercise (i.e., isometric muscle actions and/or cycle ergometry). Dr. Hug indicated that the EMGFT test should be considered as a valid tool to assess muscle function/fitness level (and to monitor changes in response to training/rehabilitation programs) only if the values are determined in a way that small changes over time or between subjects can be detected. Previous studies have shown the EMGFT test to be sensitive to training-induced changes (5), as well as differences among the superficial muscles of the quadriceps femoris (3). Therefore, the EMGFT test, as it has been examined previously (3,5) and adapted for isometric muscle actions in the present study (2), is sensitive to differences both over time and among muscles. Furthermore, we determined the mean (±SD) SE of the y-intercept (EMGFT) values (25.9 ± 28.2 W) from the study by Smith et al. (5). These SE values (5) represented a mean of 21.8 ± 22.1% of the EMGFT and were consistent with those from de Vries et al. (1) during cycle ergometry (15.5 ± 8.7% EMGFT). In addition, these values were consistent with the present study (2) during isometric leg extension (VL = 23.9 ± 23.7%, VM = 30.0 ± 25.1%, and RF = 33.8 ± 19.2% EMGFT).
Dr. Hug identified specific methodological considerations that should provide the foundation for a number of future studies that are designed to examine and improve the precision for estimating the EMGFT. These future studies should be based on an examination of the factors listed by Dr. Hug, including
I. the linearity of the relationship between the increase in EMG amplitude and time,
II. putative compensations between muscles, and
III. nonhomogeneous distribution of EMG activity within a muscle.
Furthermore, as indicated by Dr. Hug, future research should examine the "area ratio" index of Merletti et al. (4) versus the traditional methods for estimating EMGFT, the usefulness of the EMGFT for individual muscles and synergistic muscle groups, and the use of a multichannel amplifier to map activity levels and determine the sampling site.
1. de Vries HA, Moritani T, Nagata A, Magnussen K. The relation between critical power and neuromuscular fatigue as estimated from electromyographic data. Ergonomics
2. Hendrix CR, Housh TJ, Johnson GO, et al. Comparison of critical force to EMG fatigue thresholds during isometric leg extension. Med Sci Sports Exerc
3. Housh TJ, de Vries HA, Johnson GO, et al. Electromyographic fatigue thresholds of the superficial muscles of the quadriceps femoris. Eur J Appl Physiol Occup Physiol
4. Merletti R, Loconte L, Orizio C. Indices of muscle fatigue. J Electromyogr Kinesiol
5. Smith AE, Moon JR, Kendall KL, et al. The effects of β-alanine supplementation and high-intensity interval training on neuromuscular fatigue and muscle function. Eur J Appl Physiol